Melanie Bergeron

Performance Evaluation of the LabPET APD-Based Digital PET Scanner

The LabPETTM is a fully digital avalanche photodiode (APD) based PET scanner designed for state-of-the- art molecular and genomic imaging of small animals. Two versions of the scanner were evaluated, having 3.75 (LabPET4) and 7.5 cm axial FOV (LabPET8). The detectors are made of 2x2x10/12 mm3 LYSO and LGSO crystals assembled in phoswich pairs read out by an APD. After digital crystal identification, the average energy resolution is 24 plusmn 6% for LYSO and 25 plusmn 6% for LGSO. The scanner overall timing resolution is 6.6 ns for LYSO/LYSO and 10.7 ns for LGSO/LGSO coincidences after coarse timing alignment. The FBP reconstructed tangential/radial resolution is 1.3/1.4 mm FWHM (2.5/2.4 mm FWTM) at the FOV center and remains below 2.1 mm FWHM (3.6 mm FWTM) within the central 4-cm diameter FOV. MLEM reconstruction of a micro resolution phantom provided clear separation of the 1.35 mm spots and fair identification of 1 mm spots. With an energy window of 250-650 keV, the sensitivity is 1.1% for LabPET4 and 2.1% for LabPET8. The imaging capabilities of the scanner are demonstrated with in vivo images of rats and mice.

NEMA NU 4-2008 Comparison of Preclinical PET Imaging Systems

The National Electrical Manufacturers Association (NEMA) standard NU 4-2008 for performance measurements of small-animal tomographs was recently published. Before this standard, there were no standard testing procedures for preclinical PET systems, and manufacturers could not provide clear specifications similar to those available for clinical systems under NEMA NU 2-1994 and 2-2001. Consequently, performance evaluation papers used methods that were modified ad hoc from the clinical PET NEMA standard, thus making comparisons between systems difficult. Methods: We acquired NEMA NU 4-2008 performance data for a collection of commercial animal PET systems manufactured since 2000: microPET P4, microPET R4, microPET Focus 120, microPET Focus 220, Inveon, ClearPET, Mosaic HP, Argus (formerly eXplore Vista), VrPET, LabPET 8, and LabPET 12. The data included spatial resolution, counting-rate performance, scatter fraction, sensitivity, and image quality and were acquired using settings for routine PET. Results: The data showed a steady improvement in system performance for newer systems as compared with first-generation systems, with notable improvements in spatial resolution and sensitivity. Conclusion: Variation in system design makes direct comparisons between systems from different vendors difficult. When considering the results from NEMA testing, one must also consider the suitability of the PET system for the specific imaging task at hand.

System Architecture of the LabPET Small Animal PET Scanner

To address modern molecular imaging requirements, a digital positron emission tomography (PET) scanner for small animals has been developed at Universite de Sherbrooke. Based on individual readout of avalanche photodiodes (APD) coupled to LYSO/LGSO phoswich detectors, the scanner supports up to 4608 channels in a 16.2 cm diameter, 11.25 cm axial field of view with an isotropic ~ 1.2 mm FWHM intrinsic spatial resolution at the center of the field of view. Custom data acquisition boards preprocess and sample APD signals at 45 MHz and compute in real time crystal identification, energy and timing information of detected events at an average sustained rate of up to 1250 raw counts per second per mm2 (10 000 cps/channel). Real time digital signal analysis also filters out events outside the pre-selected energy window with crystal granularity to eliminate Compton events and electronic noise. Retained events are then merged into a single stream through a real-time sorting tree, at which end prompt and delayed coincidences are extracted. A single Firewire link handles both control and data transfers with a host computer. The LabPET features four data recording modes, giving the user the choice to retain data for research or to minimize file size for high coincidence count rate and imaging purposes. The electronic system also supports time synchronized data insertion for flags such as vital signs used in gated image reconstruction. Aside from data acquisition, hardware can generate live energy and discrimination spectra suitable for fast, automatic channel calibration.

Timing improvement by low-pass filtering and linear interpolation for the LabPET TM scanner

Digital processing for positron emission tomography (PET) scanners commonly relies on low frequency sampling (≪65 MHz) for reducing power consumption. Timestamps must then be interpolated between samples to achieve adequate time resolution for coincidence detection of annihilation radiation. A low-pass filter based interpolation algorithm adding up to 31 samples between original samples was designed to improve both the energy and timing resolution of the LabPETTM scanner. An energy resolution refinement of ˜2 bits can be achieved with such a technique. The better estimation of triggering threshold leads to a more accurate timestamp generation. Timestamp accuracy was investigated as a function of trigger level (5-50% of maximum value). With the trigger threshold set at 20%, coincidence time resolution of ˜5.0 ns for LYSO-LYSO and ˜9.6 ns for LGSO-LGSO are obtained. A real time implementation of the algorithm was achieved in a Xilinx FPGA.

Real Time Coincidence Detection Engine for High Count Rate Timestamp Based PET

Coincidence engines follow two main implementation flows: timestamp based systems and AND-gate based systems. The latter have been more widespread in recent years because of its lower cost and high efficiency. However, they are highly dependent on the selected electronic components, they have limited flexibility once assembled and they are customized to fit a specific scanners geometry. Timestamp based systems are gathering more attention lately, especially with high channel count fully digital systems. These new systems must however cope with important singles count rates. One option is to record every detected event and postpone coincidence detection offline. For daily use systems, a real time engine is preferable because it dramatically reduces data volume and hence image preprocessing time and raw data management. This paper presents the timestamp based coincidence engine for the LabPET¿, a small animal PET scanner with up to 4608 individual readout avalanche photodiode channels. The engine can handle up to 100 million single events per second and has extensive flexibility because it resides in programmable logic devices. It can be adapted for any detector geometry or channel count, can be ported to newer, faster programmable devices and can have extra modules added to take advantage of scanner-specific features. Finally, the user can select between full processing mode for imaging protocols and minimum processing mode to study different approaches for coincidence detection with offline software.

Timing Improvement by Low-Pass Filtering and Linear Interpolation for the LabPET Scanner

Digital processing for positron emission tomography (PET) scanners commonly relies on low frequency sampling (MHz) to reduce power consumption. Timestamps must then be interpolated between samples to achieve adequate time resolution for coincidence detection of annihilation radiation. A low-pass filter based interpolation algorithm adding up to 31 samples between original samples was designed to improve timing resolution of the LabPET scanner. A 2-bit refinement in the determination of the pulse maximum amplitude leads to a better estimation of the triggering threshold, which in turn enables a more accurate timestamp generation. Timestamp accuracy was investigated as a function of trigger level (15%-50% of maximum value). With the trigger threshold set at 20%, coincidence time resolution of ns for LYSO-LYSO and ns for LGSO-LGSO are obtained. A real time implementation of the algorithm was achieved in a Xilinx FPGA.

A Sub-Nanosecond Time Interval Detection System Using FPGA Embedded I/O Resources

The Time to Digital Converter (TDC) concept is quite useful to obtain crucial timing information for nuclear radiation detection such as PET imaging applications. The high resolution nature of TDCs makes them sensitive to process and temperature variations. Thus, a calibration procedure must often be performed to improve measurements. Moreover, field programmable gate array (FPGA)-based TDC exacerbates this problem because the transistor topology is fixed on the fabric for low cost purposes. A Sub-Nanosecond Time Interval Detection System, able to overcome process and temperature (PT) variations, was designed and implemented in an FPGA. Unlike other FPGA-based TDCs, this new solution uses embedded PT invariant digital delay lines and deserializers included in I/O ports, along with a stable clock oscillator resulting in low logic usage. The proposed design consists of oversampling digital signals to enable the creation of absolute timestamps down to 75 ps resolution (31.85 psRMS). As a proof of concept, this paper reports timing resolution down to 321.5 ps.

LabPET II, an APD-based PET detector module with counting CT imaging capability

CT imaging is currently the standard modality to provide anatomical reference in PET molecular imaging. Since both PET and CT rely on detecting radiation to generate images, it would make sense to use the same detection system for data acquisition. Merging PET and CT hardware imposes stringent requirements on detectors, including wide dynamic range with high signal-to-noise ratio for good energy resolution in both modalities, high pixellisation for high spatial resolution, and very high count rate capabilities. The APD-based LabPET II module is proposed as the building block for a truly combined PET/CT scanner. The module is made of two 4 × 8 APD pixel monolithic arrays mounted side by side unto a custom ceramic holder, with each element having an active area of 1.1 × 1.1 mm2 at a 1.2 mm pitch, coupled to a 12-mm high LYSO scintillator block array. While a previous version of the module was made of pyramidal shaped crystals (1.35 × 1.35 / 1.2 × 1.2 mm2, top/bottom), a recent version was designed with a simpler rectangular geometry (1.2 × 1.2 mm2), better reflective material optimizing the shift of refractive index at crystal interface, and enhanced APD quantum efficiency to improve intrinsic detector performance. Mean energy resolution was improved to 20 ± 1% (formerly 24 ± 1%) at 511 keV and to 41 ± 4% (formerly 48 ± 3%) at 60 keV. These intrinsic detector performance characteristics make the LabPET II module suitable for counting CT imaging with efficient energy discrimination. Initial phantom images obtained from a CT test bench demonstrated excellent contrast linearity as a function of material density and spatial resolution of 0.61 mm FWHM / 1.1 mm FWTM, corresponding to 1.3 lp/mm at MTF10% / 0.73 lp/mm at MTF50%, which allowed 0.75 mm air holes in an Ultra Micro resolution phantom to be clearly distinguished.

A Fast Crystal Identification Algorithm Applied to the LabPET™ Phoswich Detectors

Detectors based on LYSO and LGSO scintillators in a phoswich arrangement coupled to an avalanche photodiode are used in the LabPETtrade, an all-digital positron emission tomography (PET) scanner for small animal imaging developed in Sherbrooke. A Wiener filter based crystal identification (CI) algorithm achieving excellent discrimination accuracy was recently proposed for crystal identification of LYSO-LGSO phoswich detectors . This algorithm was based on estimating parameters describing the scintillation decay time constant and the light yield of events sampled at 45 MSPS. The CI process was performed by applying a threshold on the scintillation decay parameter of events. The light yield was not considered in the CI process even if it should be. We propose a 2-fold faster CI approach which takes both the scintillation decay and light yield coefficients of each crystal into consideration. The new algorithm uses the previous Wiener filter based algorithm as a calibration process in order to evaluate the model of each individual crystal. The DAQ chain model as a priori knowledge is then incorporated into the model of each crystal and the output signal is estimated. The CI is performed by evaluating a single parameter representing the percentage contribution of each crystal characteristics in the event signal. The CI algorithm demonstrated a discrimination rate accuracy for LYSO-LGSO LabPET detectors and for LSO-GSO crystals in phoswich arrangement for 511 keV events. Although a calibration is required, the real-time implementation of the new CI algorithm needs 2 times less direct operations. An FPGA clocked at 400 MHz can process up to 25 M events/sec with such an algorithm.

New UV-enhanced, ultra-low noise silicon avalanche photodiode for radiation detection and medical imaging

New fast, ultra low-noise UV-enhanced avalanche photodiodes (APD) were recently developed by Excelitas Technologies (formerly PerkinElmer Optoelectronics, Vaudreuil, QC, Canada). for use with scintillation crystals. The novel epitaxial type “buried junction” APD structure was designed for detecting photons with wavelengths shorter than ∼600 nm at high gain with very low dark current. It typically exhibits extremely low noise level (<0.1 pA/√Hz per mm2) up to multiplication gains of 200–300. The photodetecting performance of the UV-enhanced APDs with scintillators of potential interest in the fields of high-energy physics and medical imaging is presented. Energy resolution under 8% at 662 keV and subnanosecond timing resolution for annihilation radiation can be reached with LGSO 90% Lu (45 ns).